The present invention relates to Au/Fe2O3 catalyst materials made from a particulate, co-catalytically active Fe2O3 support material with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, various processes for their production and their use, particularly for selective low-temperature CO oxidation in reformate hydrogen.
The CO content in reformate hydrogen from a hydrocarbon reformer is about 5,000 ppm or over 10,000 ppm to 20,000 ppm immediately downstream of a methanol reformer. When using such a reformate hydrogen as combustible gas in polymer-electrolyte-membrane (PEM) fuel cells, this CO must be reduced almost completely, that is to about 30 ppm maximum not to poison the Pt/Ruxe2x80x94C anodes of the PEM fuel cell conventionally used. To reduce the CO content in reformate hydrogen, there are several chemical engineering concepts, of which selective CO oxidation is currently preferred for mobile applications and small stationary plants for reasons relating to cost and selectivity, but also because of the comparatively high space-time yield.
This oxidative CO removal is traditionally carried out in a multi-stage reactor by means of known high-temperature catalysts, for example Pt/Al2O3, at 200xc2x0 C. The control of such a reactor system for continuously guaranteeing a residual CO content of about 30 ppm at different load states of the fuel cell is however extremely expensive and complicated. One of the main reasons for this, which occurs particularly during transfer to weak loads with larger residence times associated therewith, is the retro-shift reaction (3) competing with the reaction equations (1) and (2) shown below, and which has to be repressed, for example by rapid increase of oxygen supply while reducing the required selectivity.
CO+xc2xdO2xe2x86x92CO2xe2x80x83xe2x80x83(1)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(2)
CO2+H2xe2x86x92CO+H2Oxe2x80x83xe2x80x83(3)
Catalyst materials have been developed, in which the Pt has been replaced by Ru or a different Pt group metal, and which have the same activity and selectivity as the traditional Pt/Al2O3 catalyst material in the temperature range from 120 to 150xc2x0 C. at comparable noble metal content.
For reasons relating to kinetics and process technology, it is advantageous to allow CO coarse cleaning to proceed in the temperature range from 190 to 230xc2x0 C. in a fixed bed reactor operating as isothermally as possible and filled with traditional Pt/Al2O3 pellets. The second or last cleaning stage (CO fine cleaning at CO starting contents of 1,000 to 2,000 ppm) is then carried out at considerably lower temperatures, for example at 120xc2x0 C., using the above-mentioned catalyst materials.
Furthermore, it has been proposed to shift the CO fine cleaning to the working region of the PEM fuel cell, that is at temperatures up to 80xc2x0 C., but for which a low-temperature CO oxidation catalyst is required.
It is known that metal oxide-supported Au catalysts show high catalytic activity during low-temperature oxidation of CO even in reducing atmosphere. Hence, it can be seen from Journal of Catalysis 168 (1997) 125-127, that an Au catalyst (Au/MnOx catalyst) supported on manganese oxides may be used for selective oxidation of CO in hydrogen. The production of the Au/MnOx catalyst is effected by coprecipitation of an aqueous solution of tetrachloroauric acid and manganese nitrate with an aqueous lithium carbonate solution, drying and calcining of the coprecipitate in air at 300xc2x0 C. The calcined sample thus consists mainly of metallic gold particles and MnCO3. After measuring the catalytic activity for CO oxidation in hydrogen for one day, decomposition of MnCO3 occurred with formation of crystalline manganese oxides, MnO, Mn3O4 and Mn2O3. In addition, there was sintering of the gold particles, wherein an average particle diameter of 2.8 nm was obtained. However, the CO conversion rate of such a catalyst material is relatively low and not satisfactory for practical application.
Applied Catalysis A: General 134 (1996) 275-283 reports on the low-temperature water gas shift reaction on Au/Fe2O3 catalysts produced by coprecipitation. It can be seen from this that a higher catalytic activity results with smaller gold particle diameter. The CO conversion rate of an Au/Fe2O3 catalyst material produced by coprecipitation is however likewise not satisfactory.
German Offenlegungsschrift 4 238 640 describes Au/Fe2O3 catalysts for hydrogenating CO and CO2, which likewise are produced by mixed precipitation of a gold compound and an iron salt.
The object of the present invention is to provide an Au/Fe2O3 catalyst material having increased activity and selectivity, particularly for low-temperature CO oxidation, and adequate long-term stability, and processes for its production.
This object is achieved by a catalyst material according to claims 1 and 3 and processes according to claims 7, 8 and 9. Advantageous or preferred embodiments of the inventive object are given in the sub-claims.
Accordingly, the object of the invention is an Au/Fe2O3 catalyst material made from a particulate, co-catalytically active Fe2O3 support material with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, which can be obtained by
a) reacting a water-soluble Fe(III) salt in an aqueous medium with a base,
b) impregnating the still moist hydroxide gel thus formed with a solution of a water-soluble Au compound to deposit complexed Au clusters on the surface of the hydroxide gel,
c) removing water from the suspension of the reaction product thus formed, and
d) subjecting the dried reaction product to calcining at temperatures between 350 and 700xc2x0 C.
According to a preferred embodiment, this catalyst material also contains at least one Fe2O3 sinter inhibitor selected from Al2O3, Cr2O3 and MgO.
The object of the invention is also an Au/Fe2O3 catalyst material made from a particulate, co-catalytically active Fe2O3 support material containing at least one Fe2O3 sinter inhibitor selected from Al2O3, Cr2O3 and MgO and with metallic Au clusters deposited thereon which have a diameter of less than 4.5 nm, which can be obtained by:
i) simultaneously reacting a water-soluble Fe(III) salt, at least one water-soluble salt of Al, Cr, Mg and a water-soluble Au compound in an aqueous medium with a base,
ii) removing water from the suspension of the reaction product thus formed, and
iii) subjecting the dried reaction product to calcining at temperatures between 350 and 700xc2x0 C.
The catalyst material of the invention preferably contains 2-8 wt. % Au, since the best results are obtained with such a gold deposit.
Furthermore, it is desirable that the catalyst material of the invention has as high as possible specific surface area, preferably of at least 50 m2/g according to the BET method. Furthermore, the Au clusters in the catalyst material of the invention have as high as possible a degree of dispersion, so that the Au clusters preferably have a diameter of less than 4 nm, also preferably of 1-3 nm.
A high specific oxide surface area and a high degree of dispersion for the Au clusters are particularly advantageous as regards kinetic points of view, since the step determining the reaction rate during CO oxidation takes place on the gold-iron oxide boundary. The degree of dispersion of the gold is therefore very important with regard to the CO conversion rate for the same Au deposit.
Regarding the CO selectivity of the catalyst materials of the invention, it has been shown that the selectivity increases for a temperature reduction from, for example 80 to 20xc2x0 C. This can be explained in that at lower temperatures CO is generally absorbed more strongly than H2. However, the rate of CO oxidation also drops with a reduction in temperature.
The Au/Fe2O3 catalyst materials of the invention show an excellent long-term stability. For example the catalyst material of the invention shows no change on one-week long storage under real reformer gas atmosphere with traces of oxygen at 80xc2x0 C. The presence of 0.3 to 1% oxygen in the reformer gas suppresses the reduction of Fe2O3 to form Fe3O4 and the formation of FeCO3.
Investigations have shown that the CO oxidation activity of the Au/Fe2O3 catalyst of the invention is higher by at least a factor 50, for comparable gold particle size between 2.5 and 4.5 nm, than for the known Au/MnOx catalyst (see also examples).
In one embodiment of the process of the invention, the catalyst material is not produced by coprecipitation, but a reaction of a water-soluble Fe(III) salt is initially effected in an aqueous medium with a base with formation of an iron oxide precursor, namely an iron hydroxide gel, wherein in a second step immediately thereafter, the still moist hydroxide gel is impregnated with a solution of a water-soluble Au compound to deposit complexed Au clusters on the surface of the hydroxide gel in the finest distribution. After removing water, the dried reaction product is then subjected to calcining at temperatures between 350 and 700xc2x0 C.
The production process of the invention permits a better, that is independent, control of the optimised pre-structures of the two reaction components. Hence, for example during the first precipitation by suitable temperature control via the grain growth rate of Fe(O) (OH)x precursor matrix, the content of surface hydroxyl groups and the water adsorbates may be adjusted not only in the hydroxide gel itself, but in the end in the pre-dried end product. Following on from that there is deposition using the dissociated, anionic Au complex, for example in the form of an [Au(Cl)4-z(OH)z]xe2x88x92 complex when using tetrachloroauric acid as the water-soluble Au compound.
According to the invention, much smaller Au clusters having an average diameter of less than 4.5 nm, in particular between 1 and 3 nm, can be fixed on the Fe2O3 support material by this process of sequential precipitation than by the known coprecipitation, in which at best gold islands having a diameter of about 4.5 nm are obtained. The increased degree of dispersion of the gold achieved according to the invention facilitates a CO conversion increase per gram of gold by a factor of 3 to 5.
According to a modified embodiment of the process of the invention described above, the first step of conversion of a water-soluble Fe(III) salt takes place in the presence of at least one water-soluble salt of Al, Cr or Mg in order to obtain a catalyst material which also contains at least one Fe2O3 sinter inhibitor selected from Al2O3, Cr2O3 and MgO.
In a third embodiment, the Au/Fe2O3 catalyst material containing at least one Fe2O3 sinter inhibitor selected from Al2O3, Cr2O3 and MgO is produced according to a process, which comprises the following steps:
i) simultaneously reacting a water-soluble Fe(III) salt, at least one water-soluble salt of Al, Cr, Mg and a water-soluble Au compound in an aqueous medium with a base,
ii) removing water from the suspension of the reaction product thus formed, and
iii) subjecting the dried reaction product to calcining at temperatures between 350 and 700xc2x0 C.
The effect of the oxides Al2O3, Cr2O3 or MgO, which have grown into the Fe2O3 crystal matrix and are formed after calcining, consists in preventing the slow sintering of haematite (xcex1-Fe2O3) or magnetite (Fe3O4) substrate and the migration and coagulation of the gold clusters during use of the catalyst material. The use of MgO as xe2x80x9cspacerxe2x80x9d is thus particularly preferred according to the invention, since the two Fe and Mg oxide precursors thus do not exist separately from one another during the production of the catalyst material, but as an Mgxe2x80x94Fe compound, for example as Mg6Fe2CO3(OH)16.4H2O (pyroaurite), together with amorphous Fe2O3. A very homogeneous mixture of the two oxides is thus achieved during calcining and the xe2x80x9cspacerxe2x80x9d effect of MgO on the Fe2O3 or on the MgFe2O4 precursor is maximised. At the same time, the mobility of the Au particles on the oxidic surface is thus restricted even during the heating time of the calcining step, as a result of which very small gold clusters are preserved. Furthermore, it may be assumed that the amorphous MgO increases the catalytic synergistic effect of molecular oxygen promotion or cleavage on the Fe2O3 surface. Finally, the carbon dioxide, which escapes as gas during calcining at about 350-400xc2x0 C., effects the formation of a secondary gas pore structure, which is desirable in the subsequent formation of catalyst pellets or in the production of a pressed catalyst insert sheet.
In the process of the invention, the precipitation and impregnation steps are preferably carried out at temperatures of 40-95xc2x0 C., also preferably at 60-85xc2x0 C.
The pH value in the precipitation and impregnation steps is preferably 6-10, also preferably 7-9.
Suitable bases are known metal hydroxides and/or metal carbonates, wherein preferably NaOH and/or Na2CO3, in particular Na2CO3, are used.
The water-soluble salts of Al, Cr or Mg are preferably used in a proportion of 0.1-3.0 moles, also preferably 0.1-1.0 mole, and still further preferably 0.1-0.5 mole, per mole of Fe.
Suitable water-soluble gold compounds are, for example tetrachloroauric acid or tetranitratoauric acid, wherein tetrachloroauric acid is particularly preferred. Fe(NO3)3 is preferably used as the water-soluble Fe(III) salt, and may alternatively contain water of crystallisation.
Calcining is effected suitably at temperatures between 350 and 700xc2x0 C., preferably between 350 and 500xc2x0 C., also preferably between 350 and 400xc2x0 C., wherein the last-mentioned temperature range is used particularly when none of the sinter inhibitors mentioned are used.
The catalyst material of the invention is suitable, for example for selective CO oxidation in reformate hydrogen, for methanisation, for CO conversion or for oxidative removal of CO and of hydrocarbons from air. The use for selective low-temperature CO oxidation in reformate hydrogen for PEM fuel cells is particularly preferred. The catalyst material of the invention may thus be processed to form pellets according to traditional processes or be pressed to form a catalyst insert sheet.